The Contractor submitted his forecast by means of a contractors communication can the Project Manager reject the contractors communication? Can a contractors communication be rejected and what is the reasons for this?
What piece of paper this is submitted on is irrelevant. As long as it complies with 13.1 and 13.7 (i.e. in writing and separate) then there is nothing to reject here. Anyway, under NEC3 and the ECC, the Project Manager has to make their own assessment of Defined Cost whether the Contractor has submitted an application anyway so again not sure what there is to reject here.
Are you referring to the forecast of the total Defined Cost under clause 20.4 of Main Option C? If that is the case, then there is no stated action by the Project Manager once it has been submitted.
Forecasts are also required to be prepared ‘in consultation with the Project Manager’ so there shouldn’t be anything surprising to him in the submission.